1. Field of the Invention
The present invention relates to a power source apparatus of computer, display and other computer peripheral devices.
2. Background of the Invention
Recently, in the computer and computer peripheral device market, a power-saving function for decreasing the power consumption of these devices when not in the energized state is desired. In these devices, a switching type power supply apparatus is used. Suppressing the power consumption by several watts by the power-saving function, requires a reduction in the power consumption at the primary side.
Such a power supply apparatus is shown in FIG. 5. FIG. 5 shows a circuit diagram of a conventional switching type power supply device; a stabilizing control circuit for stabilizing the output voltage is omitted.
As shown in FIG. 5, a power supply rectifying diode 1 rectifies an alternating-current (AC) voltage into a direct-current (DC) voltage, and a smoothing capacitor 2 smooths the rectified voltage. A starting resistor 3 supplies this rectified and smoothed voltage to a power supply control circuit 4. The power supply control circuit 4 issues pulse signals, and the base of a switching transistor 5 is connected to this power supply control circuit 4. The primary winding 6a of a power transformer 6 is connected to the collector of the switching transistor 5, thereby supplying the rectified and smoothed voltage. The power transformer 6 possesses secondary winding 6b and tertiary winding 6c aside from the primary winding 6a. A pulse voltage generated in the tertiary winding 6c is rectified and smoothed by a diode 7 and a capacitor 8, and its output voltage is supplied to both ends of the power supply control circuit 4. The pulse voltage generated in the secondary winding 6b is rectified and smoothed by a diode 9 and a capacitor 10, thereby yielding an output voltage.
Explaining the operation, first, when an AC voltage is fed to a terminal a, the AC voltage is rectified and smoothed by the power supply rectifying diode 1 and smoothing capacitor 2, and the voltage at point b is raised. Along with the elevation of the voltage at point b, a voltage is supplied to the power supply control circuit 4 through the starting resistor 3. When the voltage at the power supply terminal c of the power supply control circuit 4 reaches a specific voltage, the power supply control circuit 4 issues a pulse signal, and this pulse signal puts the switching transistor 5 into switching operation. By the switching operation of the switching transistor 5, a pulse voltage is generated in the primary winding 6a of the power transistor 6. As a result, a pulse voltage is generated in the secondary winding 6b and tertiary winding 6c.
The pulse voltage generated in the secondary winding 6b is rectified and smoothed by the diode 9 and capacitor 10, and a DC voltage is supplied to a device (not shown) connected to an output terminal d. The pulse voltage generated in the tertiary winding 6c is rectified and smoothed by the diode 7 and capacitor 8, and a DC voltage is supplied to the power supply terminal c of the power supply control circuit 4. That is, the power source of the power supply control circuit 4 is supplied through the starting resistor 3 at the beginning when AC voltage is applied to the terminal a. After the switching operation of the switching transistor 5 has begun, it is supplied through the tertiary winding 6c of the power transformer 6. However, a current is also flowing in the starting resistor 3 and the electric power is consumed.
For example, suppose AC 240 V is fed to the terminal a, the voltage of the power supply terminal c of the power supply control circuit 4 is 20 V and the resistance value of the starting resistor 3 is 50 k.OMEGA.. Ignoring the voltage drop of the power supply rectifying diode 1 and the like, it follows that EQU ((.sqroot./2.times.240 V)-20 V).sup.2 .div.50 K.OMEGA.=2.04 W
or about 2 W is always consumed by the starting resistor 3. Although the power loss may be reduced by increasing the resistance value of the starting resistor 3, it takes a longer time until the power supply control circuit 4 is actuated after an AC voltage is applied to the terminal a, and therefore it cannot be increased too much.
In the power-saving function for reducing the power consumption, the power consumption of the device is curtailed by decreasing the power consumption of the device connected to the output terminal d. It is, however, demanded to decrease the power consumption to 5 W to 8 W or less, and the power loss in the starting resistor 3 cannot be ignored.
Thus, in the active state of power supply, the problem was that the starting resistor 3 had a power loss of several watts.